The present invention generally relates to planarization compositions for lithographic processes, and in particular, to lithographic processes using non-polymeric amorphous materials as a planarizing layer.
Progress in nanotechnology depends on advances in lithography and post-lithographic processing techniques. One such technique, known as reverse-tone processing (RTP), can be used to create unique nanostructures and extend the utility and process latitude of established lithographic techniques. This particular application can benefit from improvement with respect to planarization layers.
As shown in FIG. 1, the RTP process 10 generally includes casting a topcoat film 12, typically by spin-coating, onto a surface 14 comprised of lithographic features 16, i.e., having various topographic features. The topcoat can form a planarizing film 12′ or non-planar film 12 depending on the material properties and deposition. Ideally, the deposited topcoat film completely fills all the gaps 18 between those features and forms a planar surface 20 as shown in the right figure (labeled Planar Film) regardless of the topography provided by the underlying lithographic features. In addition, the composition of the topcoat film should be selected to provide high plasma etch selectivity relative to the underlying lithographic features.
After the topcoat film is cast, a plasma etch process is used to remove a portion of the topcoat film and is stopped when the top surface of the underlying lithographic features 16 are exposed. The plasma chemistry is then changed to one that selectively etches the lithographic features (and any associated under layer) with respect to the planarizing film. After this step, the tone of the features on the substrate has been reversed and is opposite to those that were lithographically formed. Depending on the application, the reverse tone features can possess greater height-to-width aspect ratios than the original lithographically formed features.
The many technical and economic advantages of using spin-coating to cast films are known to those skilled in the art. During spin-coat for RTP, however, surface forces and fluid dynamical effects coupled with the underlying topography 16 can result in a more conformal-like coating leading to a non-planar topcoat. Subsequent etching of this non-planar film can result in poor pattern transfer as shown.
It has been found that the degree of planarization over topography such as that used in RTP depends on the aspect ratio and lateral density of the topography and on the thickness of the planarizing film. In general, tall and narrow features are more difficult to planarize, and the degree of planarization decreases as the distance between topographic features increases and as the film thickness approaches the height of the topography (Stillwagon et. al., Solid State Technol. 30(6), 67-71 (1987)). This implies that it will be difficult to planarize the space between isolated lines, especially when a thin planarizing layer is desired.
Various techniques have been attempted to provide planar coatings. For example, planarization via thermal reflow of polymeric materials has been reported (Stillwagon et. al., Solid State Technol. 30(6), 67-71 (1987)). In this technique, a polymer resin is cast in a volatile solvent and applied via spin coating. The resulting polymer film is heated above its glass transition temperature (Tg) to reduce the viscosity of the film so that surface forces will induce lateral flow and promote planarization. Unfortunately, the viscosity of polymer films do not drop significantly when heating at Tg and acceptable levels of planarization typically require high bake temperatures and/or long bake times.
A variety of other planarization approaches have also been developed. These include etch-back planarization, chemical mechanical planarization, imprint planarization (Lamb et. al., Solid State Technol. 46(10), 53-56 (2003)), and spin coating of photopolymerizable non-volatile monomers (Lin. et. al., J. Micro/Nanolith, MEMS MOEMS 7(2), 023008 (April-June 2008). Unfortunately, these approaches require special tooling and/or additional steps beyond the simple, mature, and ubiquitous spin-coating and heating techniques.
In view of the foregoing, there is a need in the art for planarization materials and methods that can be applied over topography that overcome the limitations of thermal reflow polymers and the complications and/or expense of additional tooling and/or processing steps.